Print center should be the first x an y movement after homing. Look for the G28 instruction, which homes the printer, then look for the next X and Y movement instruction - that should be to move the head to the center of the print bed ready for the print to begin.

Code:
M104 S195 ; set temperature
G21 ; set units to mm
G90 ; absolute distance mode
M92 E882 ; E axis steps per mm
M301 W125
G1 Z5 F200 ; Set Z to 5mm
G28 ; home
M109 S185 ; Set extruder temp
G1 X5 Y7 F9000 ; Set x to 5; Y to 7 mm
M109 S195 ; wait for temperature to be reached
G90 ; use absolute coordinates
G21 ; set units to millimeters
G92 E0 ; reset extrusion distance
M83 ; use relative distances for extrusion
M107
G1 Z0.300 F7800.000
G1 X54.892 Y31.286
You can see the G28 in the example above, and the instruction to move to the center of the print bed in the last line.

Unfortunately, there generally isn't a gcode instruction that sets the print bed size (that gets sent to the printer by Slic3r in the normal course of things). Can you not measure it? If all else fails, double the X and Y values given in the move-to-center instruction.